Cogged V-belt has been conventionally used as a V-belt for CVT (continuously variable transmission) used in motorcycles, ATV (four-wheeled buggy), snowmobiles and the like. The cogged V-belt has an advantage that flexibility is excellent, and is positively used in a speed-change device with small pulley diameter. However, it was required to further enhance lateral pressure resistance and bending fatigue resistance. In those cogged V-belts, there are many cases where short fibers are added as a reinforcing material to a compression rubber layer in order to improve lateral pressure resistance of the belt. Furthermore, in a variable speed belt, since misalignment occurs due to a speed-changing operation and large compressive force is applied to a belt side surface, durability of the belt was not sufficient by only addition of short fibers.
For example, Patent Document 1 discloses a rubber V-belt with cogs, which is a belt provided with an adhesion elastic body layer having cords embedded therein and holding elastic body layers (compression rubber layer) located on upper and lower sides of the adhesion elastic body layer, in which the holding elastic body layer contains chloroprene rubber, a reinforcing filler, a metal oxide vulcanizing agent, bismaleimide, and aramid short fibers, and the aramid short fibers are arranged in a width direction of the belt. In this patent document, elastic modulus in a grain direction (an orientation direction of the short fibers) is increased by the arrangement of the aramid short fibers, thereby maintaining lateral pressure resistance and improving durability. Furthermore, it is described that because too large amount of the aramid short fibers blended remarkably deteriorates bending fatigue resistance in a belt traveling direction (extension fatigue resistance), the amount is desirably 13 vol % or less. In addition, this document does not disclose the detail of the cords (tension members).
Patent Document 2 discloses a double cogged V-belt in which tension members made of para-aramid fibers are used, belt bending stiffness is from 600 to 1,200 N/mm3 and dynamic compression spring constant in a belt width direction is 15,000 N/mm or more or static spring constant in the belt width direction is 4,000 N/mm or more, for the purpose of the preparation of a double cogged V-belt having excellent bending fatigue resistance without acceleration of fatigue of the tension members made of para-aramid fibers. It is further disclosed the use of the rubber composition for forming a lower cog formation part (compression rubber layer) in which chloroprene rubber is contained as a main rubber component and para-aramid fibers are used as short fibers. In the examples of this patent document, to the chloroprene rubber are blended carbon black, magnesium oxide, zinc oxide, a vulcanization accelerator, para-aramid short fibers and the like, but there are no disclosures of details of the vulcanization accelerator.
Patent Document 3 discloses a double cogged V-belt having rubber hardness of a tension rubber layer and a compression rubber layer of Hs (JIS A)=90 to 96° and rubber hardness of an adhesion rubber layer of Hs (JIS A)=83 to 89°, for the purpose of improving lateral pressure resistance to thereby improve high-load power transmission capability while preventing occurrence of cracks and separation of each rubber layer and cord in an initial stage. This patent document discloses that the tension rubber layer and compression rubber layer of the belt are formed of a short fiber-containing rubber containing 100 parts by weight of chloroprene rubber, from 40 to 60 parts by weight of a reinforcing filler, from 1 to 20 parts by weight of at least one metal oxide vulcanizing agent of zinc oxide, magnesium oxide and lead oxide, from 2 to 10 parts by weight of bismaleimide and aramid short fibers, and the aramid short fibers are arranged in a belt width direction. It is further described that the tension member may be any material such as nylon, Tetron, polyester or aramid fiber. It is further described that because too large amount of the aramid short fibers blended remarkably deteriorates bending fatigue resistance (extension fatigue resistance) in a lengthwise direction of the belt, the amount is desirably 13 vol % or less.
That is, those patent documents disclose a cogged V-belt using para-aramid fibers as tension members and using a short fiber-containing rubber composition containing chloroprene rubber having bismaleimide and aramid fibers blended thereto, as tension and compression rubber layers.
However, those patent documents do not refer to a power transmission belt applicable to misalignment, and do not suppose to control mechanical properties of tension members. Particularly, tension members are embedded so as to decrease elongation in a length direction of a belt and it is not supposed to control misalignment by imparting stretchability. Furthermore, it appears that those patent documents do not suppose the relationship between the misalignment and durability of a belt. Therefore, in those cogged V-belt, for the purpose of not accelerating fatigue of tension members made of para-aramid fibers and of improving bending fatigue resistance, belt bending stiffness and dynamic compression spring constant in a belt width direction or static spring constant in the belt width direction are specified. Furthermore, for the purpose of improving lateral pressure resistance to thereby improve high-load power transmission capability while preventing occurrence of cracks and separation of each rubber layer and cord in an initial stage, rubber hardness of the tension rubber layer and compression rubber layer and rubber hardness of the adhesion rubber layer are specified. In other words, those patent documents have an object to improve bending fatigue resistance or improve lateral pressure resistance (stiffness in a belt width direction), thereby improving high-load power transmission capability. Thus, to respond to the high-load power transmission, it was necessary to increase stiffness in a width direction of a belt and tensile modulus in a lengthwise direction of the belt. However, excessive increase in stiffness and tensile modulus makes it difficult to absorb compression stress by deforming in a belt width direction, at the time when the belt receives large lateral pressure from pulleys when large misalignment occurs during changing a speed. As a result, traveling time until the occurrence of pop-out that tension members jump out of a belt body is short, leading to shortening of a belt life.